Calibration of a CRT display is performed to ensure that the monitor's colors are perceptually acceptable to a given display viewer. In general, CRT calibration is initially performed in a factory environment to ensure that when consumers buy a CRT it provides accurate color settings. Subsequent recalibration is also performed to compensate for shifts in CRT display characteristics over time.
In prior art methods, the processes of calibration and recalibration are performed by measuring given light emission characteristics from the display screen. These are used to determine how to adjust settings within the computer system in order to cause the CRT to display targeted color characteristics.
One prior art method employed to perform CRT calibrations attaches an external light measuring apparatus, such as a spectraradiometer, to the display screen to measure the emission characteristics of each of the red, green, and blue phosphors of the screen for a target white point having known set of chromaticity values. These emission characteristics are translated into three tristimulus values, also referred to as chromaticity values, for each phosphor color, i.e. nine values in all. The CRT settings are then manually adjusted so as to match the tristimulus readings to the known chromaticity values for the targeted white point.
Alternately, instead of manually adjusting the settings, another prior art method utilizes the computer system's central processing unit (CPU) to perform the CRT setting adjustments. This method entails externally measuring the emission characteristics of the red, green, and blue phosphors and entering these values into the CPU. The CPU contains a color correction mechanism that compares the chromaticity of the target white point value to the measured chromaticity displayed on the screen and adjusts the CRT settings accordingly.
Another method is described in U.S. patent application Ser. No. 08,036,349 entitled, "Method and System of Achieving Accurate White Point Setting of a CRT Display", assigned to the Assignee of the present invention. This method entails first performing an initial measurement and then a calibration step. The initial measurement and calibration procedures are typically performed in the factory where the CRT is assembled.
The initial measurement step includes using an external spectraradiometer to measure tristimulus values for a given image having a know white point setting. In addition, the calibration system as disclosed in U.S. patent application Ser. No. 08,036,349 includes hardware to measure individual beam currents for each of the cathodes of the CRT. This beam current measurement was not previously performed in the prior art methods.
The measured tristimulus values along with the beam currents are used to mathematically generate nine normalized tristimulus values, which are stored in a calibration memory.
The initial factory CRT calibration step, involves calibrating the CRT to an arbitrary white point using the normalized tristimulus values. Target beam currents are calculated using the normalized tristimulus values to achieve the white point setting such that the CPU generates a digital video signal to display the white point on the CRT. These target beam currents are subsequently stored. The analog beam current is then measured and compared to the calculated target beam current for the chosen white point setting. The beam current setting is subsequently adjusted to match the target beam current for the chosen white point setting.
Performing subsequent recalibration steps outside of the factory environment using the method and system as described in U.S. patent application Ser. No. 08,036,349 is easily accomplished by re-displaying the original white point image and comparing the resulting beam currents to the original target beam currents stored in the calibration memory. The CPU makes necessary adjustments to the CRT to adjust the beam current to match the target beam current. This recalibration step is completely internal to the system due to the system and method of U.S. patent application Ser. No. 08,036,349
The present invention is a method and system that utilizes the present beam current measurements along with the normalized tristimulus values obtained from the measurement techniques developed in U.S. patent application Ser. No. 08,036,349 to further simplify the recalibration process. In addition, the method and system uses ambient display conditions, along with the CRT characteristics to facilitate matching a given CRT display characteristics to other color CRT displays.